Merlin Bird ID is a free app from the Cornell Lab of Ornithology in Ithaca, New York. It launched in 2014.
Sound ID listens to the birds around you and shows the names of the species singing in real time. Photo ID identifies a bird from a picture. Users who saw a bird but did not catch a photo can answer three or five questions about its size, colours and location and get a short list of likely matches. Each species has photos, sounds, range maps and notes.
The app is free with no ads. Users download bird packs for their region, and it then works without a signal, which helps on trails and in the countryside.